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Abstract

The spectral conversion of incident sunlight by appropriate photoluminescent materials has been a widely studied issue for improving the efficiency of photovoltaic solar energy harvesting. By using phosphors with suitable excitation/emission properties, also the light conditions for plants can be adjusted to match the absorption spectra of chlorophyll dyes, in this way increasing the photosynthetic activity of the plant. Here, we report on the application of this principle to a high plant, Spinacia oleracea. We employ a calcium strontium sulfide phosphor doped with divalent europium (Ca0.4Sr0.6S:Eu2+, CSSE) on a backlight conversion foil in photosynthesis experiments. We show that this phosphor can be used to effectively convert green to red light, centering at a wavelength of ~650 nm which overlaps the absorption peaks of chlorophyll a/b pigments. A measurement system was developed to monitor the photosynthetic activity, expressed as the CO2 assimilation rate of spinach leaves under various controlled light conditions. Results show that under identical external light supply which is rich in green photons, the CO2 assimilation rate can be enhanced by more than 25% when the actinic light is modified by the CSSE conversion foil as compared to a purely reflecting reference foil. These results show that the phosphor could be potentially applied to modify the solar spectrum by converting the green photons into photosynthetically active red photons for improved photosynthetic activity.

Calculated spectra of the effective incident light, derived from multiplication of the PPFD
spectra of the C- and R-foil, respectively, with the absorption spectrum of the
S.o. chloroplasts. The inset shows the absorption spectrum of the S.
o. chloroplasts.

(a) CO2 concentration inside the reaction cell recorded over time in the presence of
C- and R-foil, respectively. Data in (a) was adopted from Ref [25]. (b) Specific CO2 assimilation rate in the presence of C. and R-foil,
respectively, under various primary photon flux densities. The dashed lines represent fits of the
data to a Levenberg-Marquardt function, Eq. (1).

Table 2 Integrated absorbed PPFD fraction in the presence of the R-foil and the C-foil, respectively,
for selected spectral regions and relative to the total PPFD integrated over the full spectrum
(%).